Process and apparatus for the storage and transportation of liquefied gases

ABSTRACT

There is disclosed a vessel having a plurality of cryogenic tanks for the storage of liquefied gases wherein the cryogenic tanks are provided with conduit means and a heat exchange means whereby the unavoidable boil-off from a lower boiling liquefied gas stored in a first cryogenic tank means, is passed in indirect heat transfer relationship with the unavoidable boil-off from a higher boiling liquefied gas stored in a second cryogenic tank means, to re-liquefy the vapors of the higher boiling liquefied gas. Additional conduit means are provided to permit alteration of the gaseous atmosphere in the second cryogenic tank means (after un-loading its content) to the atmosphere of the liquefied gas to be subsequently stored and transported therein.

United States Patent Kniel et al.

PROCESS AND APPARATUS FOR THE- STORAGE AND TRANSPORTATION 0F LIQUEFIEDGASES Inventors: Ludwig Kniel, Scarsdale; Frederick Fussman, Bronx, bothof NY.

Assignee: The Lummus Company, Bloomfield,

Filed: Apr. 27, 1973 Appl. No.2 354,925

US. Cl. 62/50; 62/54; 62/55; 62/240 Int. Cl. Fl7c 7/02 Field of Search62/45, 47, 50. 51, 54, 62/55, 240; 114/74 A References Cited UNITEDSTATES PATENTS H1968 Vanklccf 62/55 9/1968 Williams et al. 62/55 10/1970Frijlink et a1. 62/55 X Battey 62/54 Becker 62/55 X PrimaryExaminer-Meyer Perlin Assistant E.\'aminerRonald C. Capossela Attorney,Agent, or FirmMarn & Jangarathis [57] ABSTRACT There is disclosed avessel having a plurality of cryogenic tanks for the storage ofliquefied gases wherein the cryogenic tanks are provided with conduitmeans and a heat exchange means whereby the unavoidable boil-Off from alower boiling liquefied gas stored in a first cryogenic tank means, ispassed in indirect heat transfer relationship with the unavoidableboil-off from a higher boiling liquefied gas stored in a secondcryogenic tank means, to re-liquefy the vapors Of the higher boilingliquefied gas. Additional conduit means are provided to permitalteration of the gaseous atmosphere in the second cryogenic tank means(after unloading its content) to the atmosphere of the liquefied gas tobe subsequently stored and transported therein.

39 Claims, 2 Drawing Figures PATENTEDAPRISIFRS 3,877. 240

Fig. I

Fig. 2

PROCESS AND APPARATUS FOR THE STORAGE AND TRANSPORTATION OF LIQUEFIEDGASES This invention relates to the storage of liquefied gases, and moreparticularly to novel processes and apparatus for the storage andtransportation of liquefied gases at about atmospheric pressure in avessel, such as a ship. barge or the like.

BACKGROUND OF THE INVENTION In the transportation of liquefied gases,such as natural gas, ethylene, ethane or like liquefied hydrocarbons,and ammonia or like inorganic compounds, which require either highpressures for normal ambient temperatures or low temperatures tomaintain the liquid state at about atmospheric pressure, problems couldarise especially in the co-transportation of two or more such liquefiedgases in a vessel, such as a ship, barge or the like. Generally, it hasbeen found desirable to store and transport liquefied gases, e.g.,ethylene, on a ship at about atmospheric pressure in tanks properlyinsulated (cryogenic tanks) and associated with a refrigeration plantfor condensing vapors which normally evolve from the liquefied gasduring transportation. In the transportation of liquefied natural gas(LNG), it is common to utilize vaporizing LNG, which results, interalia, from conductive heat transfer, as a fuel for the vesselspropulsion system, such as disclosed in U.S. Pat. No. 2,938,359 to Cobbet al. The use of vapors from other liquefied gases, e.g. ethylene, iseither impracticable or uneconomic-al.

In U.S. Pat. No. 2,795,937 to Sattler et al. there is described aprocess and apparatus for the storage and transportation of volatileliquids, in particular, the transportation of LNG and a second liquefiedgas, e.g., ethylene, in heat insulated tanks wherein vaporized LNG isemployed as fuel to the propulsion system of the vessel. The secondliquefied gas is maintained at a temperature low enough to eliminate thenecessity of venting the liquefied gas containing tank by expanding LNGinto heat transfer equipment positioned within the tank containing thesecond liquefied gas. Such process and apparatus is uneconomic since theuse of LNG as a liquid refrigerant by expanding same and the subsequentuse thereof as a fuel is costly. Additionally, the positioning of heattransfer equipment within the tank containing the second liquefied gasrenders such equipment inaccessible for repairs.

OBJECTS OF THE INVENTION An object of the present invention is toprovide novel processes and apparatus for the storage of liquefiedgases.

Another object of the present invention is to provide novel processesand apparatus for the storage and transportation of liquefied gases by avessel.

A further object of the present invention is to provide a novel processand apparatus for the storage and transportation of liquefied gases by avessel wherein the vapors evolving from the lower boiling liquefied gasbeing transported as a result mainly of heat leakage and varying sea andweather conditions (hereinafter sometimes referred to as the unavoidableboil-off"), is utilized as a means to liquefy the vapors (i.e.,unavoidable boiloff) evolving from the higher boiling liquefied gasbeing transported.

A still further object of the present invention is to provide novelprocesses and apparatus for the en route preparation of a cryogenic tankof any such vessel for the storage and transportation of a liquefied gasdifferent than the liquefied gas previously transported therein.

Another object of the present invention is to provide a novel processand apparatus for modifying an existing liquefied natural gas shiphaving cryogenic tanks for the storage and transportation of LNG and aliquefied gas having a higher boiling point than LNG.

' Still another object of the present invention is to provide a novelprocess and apparatus to be utilized on new liquefied gas ships havingcryogenic tanks for the storage and transportation at about atmosphericpressure of diverse liquefied gases.

A still further object of the present invention is to provide a novelsystem for transporting liquefied natural gas and ethylene from a sourcethereof to a user location.

SUMMARY OF THE INVENTION These and other objects of the presentinvention are achieved on a vessel having a plurality of cryogenic tanksfor the storage of liquefied gases by providing the cryogenic tanks withconduit means and heat exchange means whereby the unavoidable boil-offfrom the lower boiling liquefied gas stored in a first cryogenic tankmeans thereof, is passed in indirect heat transfer relationship with theunavoidable boil-off from the higher boiling liquefied gas stored in asecond cryogenic tank means, to re-liquefy the vapors of the higherboiling liquefied gas. Additional conduit means and other apparatus areprovided to permit alteration of the atmosphere in the cryogenic tank(after un-loading its contents) to the atmosphere of the liquefied gasto be subsequently stored and transported therein (as more fullyhereinafter discussed). The vessels in service at the present day fortransportation of liquefied gases are generally designed to carry from 5to 7 similarly sized tanks of the spherical or prismatic type, althoughother shapes of cryogenic tank may be employed. To facilitate anunderstanding of the present invention, a preferred emstoring theliquefied ethylene regardless of the size and number of such tanks, itbeing understood that volumetric ratios will vary depending upon theliquefied gases being transported. While the drawing includes fluidcommunication equipment, such as valves, pumps, and the like, it isunderstood that additional such equipment has been omitted from thedrawing to facilitate the description thereof and the placing of suchequipment at appropriate places are deemed to be within the scopeofthose skilled in the art.

BRIEF DESCRIPTION OF THE DRAWING The invention will now be describedwith reference to the accompanying drawings wherein like numerals areused throughout and wherein:

FIG. 1 is a schematic elevational view of a vessel illustrating thegeneral arrangement of the various tanks and associated apparatus; and

FIG. 2 is a schematic flow diagram of a preferred embodiment of theinvention.

7 DESCRIPTION OF THE INVENTION Referring now to FIG. 1, there isillustrated a vessel, generally indicated as 10, provided with fivespherical cryogenic tanks T including a re-liquefaction assembly, acargo control station, and an off-loading station, generally indicatedas 12, 14 and 16, respectively, with the reliquefaction assembly 12being disposed between the first and second tank proximate to the superstructure of the vessel 10. Such a vessel may have a capacity of, forexample, 125,0Om of LNG, with principal characteristics being an overalllength of about 900-1,000 feet, a draft of 36 feet or more, and adisplacement of 94,600 long tons at speeds of about 20 knots or more. Inaccordance with the present invention, one tank T(LE) to store liquefiedethylene and the remaining tanks T(LNG) to store liquefied natural gaswill be provided with piping or conduit configuration in fluidcommunication with the re-liquefaction apparatus 12 as more fullyhereinafter described. A preferred placement of the liquefied ethylenestorage tank T(LE) to minimize piping expenses is to position theliquefied ethylene tank proximate to the stern of the ship, however itis understood that other consideration, e.g., partial loadingconditions, order of loading and unloading of liquefied ethylene andLNG, etc., may make it desirable to position the liquefied ethylenestorage tank T(LE) amidships. All of the cryogenic tanks of the vessel10 are usually of the same constructions and heavily insulated.

Referring now to FIG. 2, there is illustrated the reliquefactionapparatus 12 with a piping configuration associated with the tank T( LE)for the storage of liquefied ethylene. A line for the loading andoff-loading of liquefied ethylene is provided with a suitable fitting(not shown) for connecting the line 20 to an appropriate dock sidefacilities (not shown). The line 20 is in fluid communication withloading line 22 under the control of valve 24, and with an unloadingpump 28 by line 30 under the control valve 32. Unloading pump 28 may belocated inside the tank, i.e., submerged, or externally of the tank.

A line 34 is provided on the top portion of the liquefied ethylene tankT( LE) under the control of valve 36 in fluid communication via lines 38and 40 under the control of valves 42 and 44 with a fractionatingcondenser, generally indicated as 46. The tank T(LE) is provided withsafety vent line 26 under the control of a safety relief valve 18 byline 34. Line 34 is in fluid communication with a line 48 under thecontrol of valve 50 with a compressor 52 and thence with line 40 by line54 under the control of valve 56. A line 58 under the control of valve60 is in fluid communication with a reboiler coil 63 positioned in thelower portion of the fractionating condenser 46 (as more fullyhereinafter discussed) and with line 40 by line 62 under the control ofvalve 64.

A vapor header or manifold, generally indicated as 66, is in fluidcommunication by conduits 68 with the remaining tanks of the vessel 10containing LNG. The header 66 is in fluid communication by line 70 underthe control of valve 72 with a heat exchange coil 74 disposed within theupper portion of the fractionating condenser 46; The outlet of the coil74 is in fluid communication by line 76 under the control of valve 78via a booster compressor 80 with a fuel manifold 82. The top of thecondenser 46 is in fluid communication by line 84 under the control ofvalve 86 with a heat exchange coil 88 disposed within the upper portionof the fractionating condenser 46 proximate to the coil 74 and thencewith line 76 by line 90 under the control of valve 92.

The header 66 is in fluid communication by line 94 under the control ofvalve 96 with the suction side of a compressor 98 with the outlettherefrom being in fluid communication with line 100. Line 100 is influid communication with line 102 and 104 under the control of valves106 and 108, respectively. Line 102 is in fluid communication with line70, and line 104 is in fluid communication with lines 110 and 112 underthe control of valves 114 and 116, respectively, with line 110 being influid communication with the liquefied ethylene tank T(LE).

The bottom of the fractionating tower 46 is in fluid communication byline via pump 122 with lines 124 and 126 under the control of valves 128and 130, respectively. Line 124 is in fluid communication with theliquefied ethylene tank T(LE) whereas the line 126 is in fluidcommunication with liquefied ethylene holding tank 132. The liquefiedethylene holding tank 132 is provided with a line 134 under the controlof valve 136 for the fluid communication with dock side facilities, asmore fully hereinafter discussed. The bottom of the ethylene holdingtank 132 is provided with a conduit 138 in fluid communication by a pump140 with line 142 under the control of valve 144. Line 142 is in fluidcommunication with line 146 and 148 under the control of valves 150 and152, respectively. Line 146 is in fluid communication with a line 154positioned within the tank T(LE) and through line 156 under the controlof valve 158 with line 34. Line 148 is in fluid communication with tankT(LE) via heat exchanger 160 through line 162 under the control of valve164.

As hereinabove discussed, the processes and apparatus of the presentinvention will be described with reference to the storage andtransportation of LNG and liquefied ethylene, although it is understoodthat the invention is also applicable to the handling of LNG and otherdiverse liquefied gases. It is understood in the shipment of a liquefiedgas that vapors will evolve as a result of the heat leakage into thecryogenic tanks from the surrounding environment as well as from themotion of the vessel in the sea (converted kinetic energy). It will beappreciated that a vessel is subjected to movement in heavy seas andwill evolve a greater proportion of vapors due to kinetic energy inputinto the cargo, than a vessel passing through a calm sea. As hereinabovedisclosed, this invention relates to the storage of liquefied gases in aplurality of cryogenic tanks positioned on a vessel, however, for thetransportation of LNG and liquefied ethylene the volumetric capacity ofthe tank(s) storing LNG is in a ratio of at least 4:1 with respect tothe volumetric capacity of the tank(s) storing liquefied ethylene.Therefore, with regard to the vessel of FIG. 1, one tank of the fivecryogenic tanks would be provided with conduit means and ancillaryequipment necessary to permit the storage of the liquefied ethylene, andre-liquefaction of the unavoidable boil-off, as hereinabove discussed.

TRANSPORTATION OF LNG AND LIQUEFIED ETHYLENE In operation, the tankT(LE) of the vessel is filled with liquefied ethylene and the remainingtanks T(LNG) of the vessel 10 are filled with LNG at pressures slightlyabove atmospheric. The ethylene vapors resulting from unavoidableboil-off are withdrawn from tank T( LE) by line 34 and passed by line 38and 40 into the fractionating condenser 46. In fractionating condenser46, the ethylene vapors are passed in indirect heat transferrelationship to natural gas (unavoidable boil-off) introduced into thecoil 74 of the condenser 46 from line 70 collected in manifold 66 fromthe tanks T( LNG) by lines 68. It will be understood that valve 72 willbe opened at this time whereas the valves 96, 106 and 116 will beclosed. The amount of unavoidable boil-off from the LNG in the tanks T(LNG) is sufficient to provide the cooling requirements to re-liquefy theunavoidable boil-off from the tank T( LE) in fractionating condenser 46with liquefied ethylene being returned by line 124 to the tank T(LE) bypump 122 in fluid communication with the bottom of the fractionatingcondenser 46 through line 120. The natural gas withdrawn from the coilby line 76 is passed as fuel by booster compressor 80 through line 82 tothe propulsion system of the vessel (not shown).

Should the liquefied gases be at ambient pressures or lower viceslightly above atmospheric pressure (which will generally be the casewhen prismatic tanks are utilized, or when a tank is incompletely filledwith liquefied gas), it is necessary to utilize the compressors 52 and98 to increase the pressure of the respective gaseous streams toovercome frictional resistance in the various lines, valves, andassociated apparatus. Utilization of the compressors will generally notbe necessary when the tanks are of the spherical type. The compressor S2is preferably of the non-lubricated reciprocating type with the suctionvolume thereof being regulated either by a variable speed transmissionor by clearance pockets and valve lifters, or through a combination ofsuch devices actuated by a pressure controller 170 in communication withthe tank T(LE) by line 172. The compressor 98 may be a compressorsimilar to the compressor 52. It is contemplated that in some instancesit may be necessary to utilize a plurality of such compressors inparallel. It is noted that in this operation the ethylene vapors are notpassed by line 58 through the reboiler coil 63 in the fractionatingcondenser 46, the use thereof being hereinafter discussed. It will befurther understood that pump 122 may not be necessary since theliquefied ethylene may usually be readily passed by gravity to theliquefied ethylene tank T( LE). Upon the vessel reaching itsdestination, the liquefied ethylene and/or LNG are unloaded in a mannerknown to those skilled in the art. Liquefied ethylene, for example, iswithdrawn by the pump 28 from the tank T(LE) by line 30 and is passedthrough lines 30 and to dock side facilities (not shown) with lines 34and 174 being in fluid communication with the facilities to provide forpressure equalization.

It is desirable to maintain the tankers in continuous service withminimal time for layovers or delays (outage time). Thus, if sufficientliquefied ethylene to fill the tank T(LE) is not available at theloading terminal, rather than permit the vessel to ship out with anempty or partially filled tank, it is desirable to take advantage ofsuch space by filling the tank T(LE) with LNG. In order to fill the tankT(LE) with LNG it is necessary to prepare the tank for such usage (byprecooling and by removal of the ethylene atmosphere), and this may beconveniently accomplished while the vessel is en route to the loadingterminal since in port preparation is wasteful of LNG. Preparation ofthe tank T(LE) for the cargo switch, either from liquefied ethylene toLNG or vice versa, involves two operations, i.e., (a) the replacement ofthe tanks atmosphere with an atmosphere of the material to betransported, and (b) the cooling or heating of the tank to a temperaturecompatible with that of the boiling point of the material to be storedto thereby reduce or avoid the so-called load flash or a negativepressure which arises when the cryogenic tank is either too warm to toocold, respectively, to receive the new material.

CONVERSION OF THE LIQUEFIED ETHYLENE TANK TO LNG STORAGE Unloadingliquefied ethylene in a manner known to those skilled in the artincludes the introduction of gaseous ethylene from a shore side sourcein fluid communication with storage tanks into the space in the tankT(LE) created by the withdrawn liquefied ethylene. After unloadingliquefied ethylene from, for example, a 25,000 cubic meter tank, therewill remain about 50 metric tons of ethylene vapors at a temperature ofabout l40F, at a pressure of about 0.5 psig. as well as a residualamount of liquefied ethylene. A predetermined amount of LNG remains inthe LNG tanks to maintain the tanks at proper pressure and temperatureconditions during the return voyage, as is known to those skilled in theart.

- On the return voyage, the unavoidable boil-off from the LNG tanks iscollected in the manifold 66 and a minor portion thereof is introducedinto tank T(LE) through line 112 and under the control of valves 116 and114, respectively, or alternately, through line 94, 100, 104 and 110 viathe compressor 98 if the pressure in T(LNG). is insufficient. Theremaining portion of the unavoidable boil-off from the LNG tanks ispassed to the coil 74 in the fractionating condenser 46 through line 70under the control of valve 72. Ethylene vapors are gradually downwardlydisplaced by the natural gas introduced into thetank T(LE) with onlylimited amount'of cross mixing effected at the interface since naturalgas which is essentially methane (with limited amounts of nitrogen) isconsiderably lighter than the ethylene vapors contained in the tankespecially upon being warmed-up by the walls of the tank. The displacedethylene vapors are withdrawn from tank T(LE) through line 154 under thecontrol of valve 158, line 156, line 34 and line 48 under the control ofvalve 50 by the compressor 52. Compressed ethylene vapors are thenintroduced by lines 54 and 40 into the condenser 46 wherein the ethylenevapors are passed in indirect heat transfer relationship with naturalgas in coil 74 to liquefy thereby the ethylene vapors. Liquefiedethylene collected in the bottom of the condenser 46 is withdrawn fromcondenser 46 by pump 122 through line and is passed by line 126 underthe control of valve 130 to the liquefied ethylene holding tank 132.Ethylene vapors'in the liquefied ethylene holding tank 132 are returnedto the compressor 52 through lines 134, 34 and 48.

During the initial phases of the purge procedure, as hereinabovediscussed, essentially pure ethylene vapors from the liquefied ethylenetank T(LE) are introduced into the fractionating condenser 46. Traceimpurities, such as methane and hydrogen in the ethylene vapors arewithdrawn as condenser overhead in line 84 and are passed through coil88 in condenser 46 to heat the gas prior to introduction into the fuelgas header 82. After purge procedure has progressed to a point whereabout 75% of the volume of tank T(LE) includes natural gas, the gaseousstream in line 154 will contain increasing amounts of methane whichnecessitates the use of the reboiler coil 63 in condenser 46 to stripthe methane from condensed liquefied ethylene accumulating in the bottomof the condenser 46.

The purge of the tank T(LE) with cold natural gas will concomitantlycool the tank and by the time a volume of natural gas at a temperatureof 235F equal to the volume of the tank has been introduced into thetank the temperature of the walls of the tank will have dropped by some6 to 8F depending of course on the heat capacity of the material ofconstruction of the tank walls and the amount of insulation. In thisregard, spherical tanks which are constructed for a slight pressureabove atmospheric will take a considerably longer time to cool down thando membrane type of tanks. To purge a tank of its ethylene vapors underatmospheric pressure will take from about 60 to 80 hours in which timesome 92 to 95% of the ethylene vapors will have been condensed into theliquid phase. The liquefied ethylene stored in liquefied ethylene.holding tank 132 will be contaminated with a little methane which is ofno consequence in view of the subsequent use of the liquefied ethylenein the tank 132, as more fully hereinafter discussed.

It will be obvious to one skilled in the art that in accordance withthis hereinabove discussed purge procedure that a tank may be cooledonly a limited number of degrees regardless of the length of time ofsuch procedure. If there were no heat losses, the tank could be cooledto within a few degrees of the temperature of the natural gas introducedinto the coil 74 in the condenser 46. However, in accordance with thepurge procedure of the present invention, the temperature of the tank T(LE) is lowered to a temperature of about 95 to 2()()F. To further reducethe temperature of the tank to about the temperature of the boilingpoint of LNG would require the introduction of LNG at a controlled rateof injection such as is the normal practice when cooling LNG tanks atthe loading terminal. Without the purge procedure of the presentinvention, the amount of LNG required to cool the tank is in the orderof about 26 metric tons depending upon the heat capacity of thematerials of construction of the tank walls and the amount ofinsulation. Such an amount of LNG to be vaporized is equal to thepropulsion fuel requirement of about 2 V2 to 3 hours of a tank shipdesigned in accordance with the above specification. Consequently, it isthus advantageous to effect such purge procedure during the returnvoyage of the vessel since vaporizing LNG may be passed to thepropulsion unit of the vessel rather than effecting the purge procedureat the loading terminal and burning the purged vapors in a flare stackincluded in the loading terminal facilities, thereby losing the fuelvalue thereof.

CONVERSlON FROM LNG STORAGE TO LlQUEFlED ETHYLENE STORAGE Should a tankof vessel previously have been used to transport LNG and is to bereadied for liquefied ethylene transport, it is necessary to purge thetank T(LE) of the natural gas and LNG as well as to heat the wallsthereof. Accordingly, liquefied ethylene from the holding tank 132 iswithdrawn through line 138 by pump 140 and is passed through line 142and 148 to heat exchanger 160 for vaporization of the liquefiedethylene. Gaseous ethylene is withdrawn from heat exchanger 160 and ispassed through line 162 under the control of valve 164 to the liquefiedethylene tank T( LE). Upon introduction into the tank T(LE), gaseousethylene is condensed in the colder atmosphere and against the coldertank walls thereby warming up both the atmosphere and tank walls. As thepressure tends to increase, gases are withdrawn by line 34 and passed tothe fuel gas header 82 via the condenser 46 and the coil 88 throughlines 38, 40, 84, 90 and 76. Upon reaching a temperature of about l70F,introduction of gaseous ethylene in line 162 is discontinued and thetank T(LE) permitted to warm up further by natural conduction ('i.e.,normal heat leakage).

Upon reaching a temperature of about l50F, the remaining portion ofliquefied ethylene in the holding tank 132 is withdrawn at a controlledrate through line 138 by pump 140 and passed through line 142 and 146under the control of valves 144 and 150, respectively, and by line 154into the tank T(LE). As the tank T(LE) becomes further warmed by heattransfer from the surrounding environment, a portion of the liquefiedethylene in the tank T(LE) becomes vaporized thereby graduallydriving-off the residual amounts of natural gas through line 34 and intothe fuel gas header 82, as hereinabove discussed. Minor quantities ofgaseous ethylene will be contained in the natural gas withdrawn from thetank T(LE) through line 34 and such minor quantities of gaseous ethyleneare condensed in the fractionating condenser 46 and returned to the tankT( LE) through line 124 by pump 122.

Liquefied ethylene is introduced into the liquefied ethylene holdingtank 132 at the outset of the contemplation of ethylene transportationin an amount or inventory sufficient to replace the natural gasatmosphere in the tank T( LE) by an ethylene atmosphere ofrelativelyhigh purity. Moreover, the atmosphere will constantly befurther purified by continued operation of compressor 52 to maintain thepressure in the tank T( LE) at a constant value upon arrival of thevessel at the on-loading facilities whereat the vapor space of the tankT( LE) is placed in fluid communication by lines 34 and 174 with shoreside liquefied ethylene storage tanks without endangering or impairingthe quality of liquefied ethylene stored therein. Thus, liquefiedethylene may be taken on board without any appreciable load flash loss"occasioned by the tank T(LE) not being in temperature equilibrium withthe liquefied ethylene being loaded.

it will be appreciated that the modifications of the pressuremaintenance or re-liquefaction system to eliminate any loss of a morevaluable cargo, e.'g., ethylene in the simultaneous transportation ofLNG and liquefied ethylene or other higher boiling hydrocarbons and tocreate a capability while the vessel is at sea to prepare for a changein liquefied gas as hereinabove described, eliminates costly delays ofthe vessel in port and saves valuable cargo by a very modicum ofadditional equipment.

SYSTEM FOR FLEET TRANSPORTATION OF LIQUEFIED GASES It will be understoodthat a viable size LNG route would require a plurality of such vesselshandling between about 3 million to 5 million tons of LNG annually forthe transportation of LNG from the source port to the terminal port.Liquefied ethylene has been shipped in small refrigerated tank ships (upto about 4,000 cubic meters) in mostly coastwise traffic over distancesin the order of 1,000 nautical miles. The exportation of LNG togetherwith the liquefied product of an ethylene plant would effect economiesin the transportation of liquefied ethylene as compared with tank shiptransportation of liquefied ethylene per se. Thus, if it were necessaryto transport about 3 million metric tons per year of LNG and about450,000 metric tons of liquefied ethylene per year between terminalsabout 7,000 nautical miles apart, a system would require a fleet of sixtank ships having the specifications hereinabove described with at leastthree tank ships modified with the hereinabove discussed reliquefactionsystem, although it is contemplated that all of the tank ships of thefleet may be so equipped. Thus, the ships could carry LNG alone, or LNGand liquefied ethylene depending upon the quantity of liquefied ethyleneavailable to be transported upon arrival of any one of the ships of thefleet at the loading terminal. Thus, if the expected quantity ofliquefied ethylene at the loading terminal would be insufficient to fillthe tank at the estimated time of arrival, the captain of the vesselwould be accordingly notified and the tank T( LE) would be subjectedduring the return voyage to the hereinabove discussed purge procedure tochange the atmosphere in the tank T(LE) to permit LNG loading.Alternately, if the tank T(LE) had stored LNG and a sufficient quantityof liquefied ethylene is available to fill the tank, the tank T(LE)could be converted to liquefied ethylene handling duty as alsohereinabove discussed.

The liquefied ethylene would be transported without loss thereof byutilizing the cold potential in the unavoidable boil-off from the LNGtanks during transportation as a result of heat leakage as well as theeffects of sea conditions.

The storage facilities at the terminals would depend on the capacity ofthe vessels, and would be nominally sized to store a multiple of about1.2 to 2.0 times the volume of a vessel. Thus, it is possible inaccordance with the system of the present invention to provide a fleetof vessels capable of handling LNG and liquefied ethylene, and to morefrequently transport the liquefied ethylene in smaller volumes therebyreducing the size of the liquefied ethylene storage facilities at theterminals to a fraction of that required if a liquefied ethylene tankship of optimal size were employed. Thus, the system of the presentinvention would be to employ a fleet of vessels equipped with thehereinabove disclosed re-liquefaction facilities, which vessels are incontinuous operation between terminals wherein one cryogenic tank ofeach vessel is capable of handling LNG or liquefied ethylene withprovisions to eliminate or minimize ethylene losses and wherein the tankso equipped may be converted to the storage of another liquefied gasduring transit of the vessel.

EXAMPLE The following table is illustrative of the operatingcharacteristics of the process of the present invention for there-liquefaction of ethylene vapors utilizing the unavoidable boil-offfrom the LNG tanks of a tanker having a capacity of 125,000 cubicmeters:

As hereinabove mentioned, the present invention is applicable to thetransportation of other combinations of liquefied gas, e.g., LNG-ethane;LNG-LPG; LNG- propane; LNG-ammonia, etc., wherein the liquefied gaseshave substantially different boiling points. It is particularlyadvantageously applicable to the transportation of a combination ofliquefied gases wherein the lower boiling liquefied gas may be used asfuel for the vessel. The lower boiling gas should be present in anamount to provide the refrigeration potential to condense theunavoidable boil-off of the higher boiling liquefied gas. With regard tothe transportation of LNG- liquefied ethylene the respective volumetriccapacity of the tank(s) should be in a ratio of at least 4:1 ashereinabove discussed, whereas with other combinations of liquefied gas,the ratios will differ. For example, the transportation of LNG-liquefiedethane would require volumetric ratios of between about 3.0:1 to 3.511whereas for an LNG-LPG system, a volumetric ratio of as low as 2:1 wouldbe satisfactory.

While the invention has been described in connection with severalexemplary embodiments thereof, it will be understood that manymodifications will be apparent to those of ordinary skill in the art;and that this application is intended to cover any adaptations orvariations thereof. Therefore, it is manifestly intended that thisinvention be only limited by the claims and the equivalents thereof.

What is claimed is:

l. A process for the re-liquefaction of a gas evolved from a firstliquefied gas stored at about atmospheric pressure in a first cryogenictank means proximate to a second cryogenic tank means storing a secondlique- 3. The process as defined in claim 2 wherein said higher boilingliquefied gas is returned to said cryogenic tank means storing saidhigher boiling liquefied gas.

4. The process as defined in claim 2 wherein said liquefied gaseswithdrawn from said respective cryogenic tank means are compressed priorto step (c).

5. The process as defined in claim 2 wherein the gas from said secondcryogenic tank means recovered from step (c) is passed to a propulsionunit of said vessel.

6. The process as defined in claim 2 wherein said second liquefied gasis LNG.

7. The process as defined in claim 6 wherein said first liquefied gas isliquefied ethylene and wherein the ratio of the volumetric capacity ofsaid second cryogenic tank means to the volumetric capacity of saidfirst cryogenic tank means is at least about 4:1.

8. The process as defined in claim 6 wherein said first liquefied gas isliquefied ethane and wherein the ratio of the volumetric capacity ofsaid second cryogenic tank means to the volumetric capacity of saidfirst cryogenic tank means is between about 3.0:] to 3.5: l.

9. The process as defined in claim 6 wherein said first liquefied gas isLPG and wherein the ratio of the volumetric capacity of said secondcryogenic tank means to the volumetric capacity of said first cryogenictank means is about 2:1. I

10. Apparatus for the re-liquefaction of a gas evolved from a firstliquefied gas stored in first cryogenic tank means proximate to a secondcryogenic tank means storing a second liquefied gas wherein saidliquefied gases are stored at about atmospheric pressure and whereinsaid liquefied gas have different boiling points which comprises:

a. first conduit means for withdrawing a gas from said first cryogenictank means;

b. a second conduit means for withdrawing a gas from said secondcryogenic tank means; and

c. heat transfer means for passing said gases in indirect heat transferrelationship to re-liquefy the gas withdrawn from the cryogenic tankmeans storing the higher boiling liquefied gas.

11. The apparatus as defined in claim 10 including a third conduit meansfor passing said liquefied gas from said heat transfer means to saidcryogenic tank means storing said higher boiling liquefied gas.

12. The apparatus as defined in claim 10 wherein said re-liquefactionapparatus is disposed on a vessel.

13. The apparatus as defined in claim 12 including compressor meansdisposed in said first and second conduit means.

14. The apparatus as defined in claim 12 including a fourth conduitmeans for the gas withdrawn from said cryogenic tank means storing alower boiling liquefied gas to .propulsion means for said vessel.

15. Apparatus for modifying a cryogenic tank on a vessel having aplurality of cryogenic tanks for storing at about atmospheric pressureliquefied gases having different boiling points wherein said modifiedtank is to store a higher boiling liquefied gas which comprises:

a. heat transfer means positioned on said vessel;

b. a first conduit means disposed between the vapor space of said one ofsaid plurality of cryogenic tanks and said heat transfer means; and

c. a second conduit means disposed between the vapor space of saidremaining tanks and said heat transfer means, said heat transfer meansproviding for the passage in indirect heat transfer relationship ofvapors in each of said conduit means.

16. The apparatus as defined in claim 15 including a third conduit meansdisposed between said heat transfer means and said one of said pluralityof tanks.

17. The apparatus as defined in claim 16 including a fourth conduitmeans disposed between said heat transfer means and a liquefied gasholding tank.

18. The apparatus as defined in claim 17 including a fifth conduit meansdisposed between said holding tank and said one of said plurality ofcryogenic tanks, and wherein a second heat transfer means is provided insaid fifth conduit means.

19. A method for modifying a cryogenic tank on a vessel having aplurality of cryogenic tanks to store at about atmospheric pressureliquefied gases having different boiling points wherein said modifiedtank is to store a higher boiling liquefied gas of said liquefied gaseswhich comprises:

a. positioning a heat transfer means on said vessel;

b. providing a first conduit means from the vapor space of said one ofsaid plurality of tanks to said heat transfer means; and

c. providing a second conduit means between the vapor space of saidremaining tanks and said heat transfer means. said heat transfer meansproviding for the passage of said gases in each of said conduit means inindirect heat transfer relationship therebetween.

20. The method as defined in claim 19 wherein a third conduit means isprovided between said heat transfer means and said one of said pluralityof tanks.

21. The method as defined in claim 20 wherein a fourth conduit means isprovided between said heat transfer means and a liquefied gas holdingtank.

22. The method as defined in claim 21 wherein a fifth conduit means isprovided between said holding tank and said one of said plurality oftanks and wherein a second heat transfer means is included in said fifthconduit means.

. 23. The method defined in claim 19 wherein a compressor means isprovided in said first and second conduit means.

24. A process for the preparation of a first cryogenic tank means forthe storage of a first liquefied gas on a vessel having a secondcryogenic tank means for the storage of said first liquefied gas whereinsaid first cryogenic tank means previously stored a second liquefied gashaving a higher boiling point and wherein residual quantities of saidsecond liquefied gas remain in said first cryogenic tank means afterunloading said second liquefied gas therefrom, which comprises:

a. withdrawing a gas evolved from the residual portions of gas in saidsecond cryogenic tank means;

b. introducing a portion of said evolved gas of step (a) into said firstcryogenic tank means;

c. withdrawing a gas from said first cryogenic tank means;

(1. passing said gas of step (c) in indirect heat transfer relationshipwith the remaining portion of gas from step (a) to liquefy said gas ofstep (c); and

e. passing said liquefied gas recovered from step (d) to a storageholding zone.

25. The process as defined in claim 24 wherein said first liquefied gasis LNG.

26. The process as defined in claim wherein said second liquefied gas isliquefied ethylene. and wherein the ratio of the volumetric capacity ofsaid second cryogenic tank means to thevolumetric capacity of said firstcryogenic tank means is'at least about 4:1. a

27. The process as defined in claim 24 wherein said preparation iseffected during transit of said. vessel;

28. Apparatus for the preparation of a first cryogenic means for thestorage of a first liquefied gas on a vessel having a second cryogenictank means for the storage of said first liquefied gas wherein saidfirst cryogenic tank means previously stored a second liquefied gashaving a boiling point higher than said first liquefied gas whichcomprises:

a. a first conduit means for withdrawing a gas from said first cryogenictank means;

b. a second conduit means for withdrawing a gas from said secondcryogenic tank means;

c. a third conduit means for passing a portion of said gas withdrawnfrom said second cryogenic tank means to said first cryogenic tankmeans; and

d. heat transfer means for passing in indirect heat transferrelationship the gas in said first conduit means and the remainingportion of gas in said second conduit means to liquefy said gas in saidfirst conduit means.

29. The apparatus as defined in claim 28 including a fourth conduitmeans for passing liquefied gas to a storage tank.

30. The apparatus as defined in claim 28 wherein the ratio of thevolumetric capacity of said second cryogenic tank means to thevolumetric capacity of said first cryogenic tank means is at least about4:1 and wherein said first and second liquefied gases are LNG andliquefied ethylene, respectively.

31. A process for the preparation of a first cryogenic tank means forthe storage of a first liquefied gas on a vessel having a secondcryogenic tank means for the storage of a second liquefied gas having aboiling point lower than said first liquefied gas wherein said firstcryogenic tank means previously stored said first liquefied gas, whichcomprises:

a. introducing into said first cryogenic tank means a gas formed fromsaid first liquefied gas;

b. withdrawing a gas from said first cryogenic tank means and passingsaid gas in indirect heat transfer relationship with a gas evolved fromsaid second liquefied gas in said second cryogenic tank means tocondense higher boiling gas contained therein; and

c. discontinuing the flow of said gas into said first cryogenic tankmeans after a predetermined period of time and thereafter introducing afirst portion of said first liquefied gas into said first cryogenic tankmeans.

32. The process as defined in claim 31 wherein said first and secondliquefied gases are liquefied ethylene and LNG, respectively, andwherein the ratio of the volumetric capacity of said second cryogenictank means to the volumetric capacity of said first cryogenic tank meansis at least about 4:1.

33. Apparatus for the preparation of a first cryogenic tank means forthe storage of a first liquefied gas on a vessel having a secondcryogenic tank means for the storage of a second liquefied gas having alower boiling point than said first liquefied gas and wherein said firstcryogenic tank means previously stored said second liquefied gas whichcomprises:

a. a storage tank on said vessel for said first liquefied gas; .1

b. a first conduit means for introducing into said first cryogenic tankmeans a gas from said storage tank;

c. a second conduit means for withdrawing gas from 7 said firstcryogenic tank means;

d. a third conduit means for withdrawing a gas from said secondcryogenic tank means; and

e. a heat transfer means for passing in indirect heat transferrelationship the gas in each of said second and third conduit means tocondense any higher boiling gas in said second conduit means.

34. The apparatus as defined in claim 33 wherein the ratio of thevolumetric capacity of said second cryogenic tank means to thevolumetric capacity of said first cryogenic tank means is at least about4:1 and wherein said first and second liquefied gases are liquefiedethylene and LNG, respectively.

35. A method of transporting a first liquefied gas from a loadingterminal to an unloading terminal utilizing a plurality of vesselshaving a first cryogenic tank means for said first liquefied gas and asecond cryogenic tank means for a second liquefied gas having a boilingpoint higher than said first liquefied gas which comprises:

a. determining the quantity of said second liquefied gas to be stored atsaid loading terminal at the estimated time of arrival of said vessel;and

b. preparing en route said second cryogenic tank means for the loadingof said first liquefied gas should the amount of said second liquefiedgas be insufficient to fill said second cryogenic tank means, thepreparation of said second cryogenic tank means including theintroduction of a portion of natural gas in said first cryogenic tankmeans into said second cryogenic tank means and the reliquefaction ofhigher boiling gas withdrawn from said second cryogenic tank means bythe passage of the gas withdrawn from said second cryogenic tank meansin indirect heat transfer relationship to a portion of natural gaswithdrawn from said second cryogenic tank means.

36. The method as defined in claim 35 wherein said first and secondliquefied gases are LNG and liquefied ethylene, and wherein the ratio ofthe volumetric capacity of said first cryogenic tank means to thevolumetric capacity of said second cryogenic tank means is at leastabout 4:1.

37. A method of transporting a first liquefied gas and a secondliquefied gas having a boiling point higher than said first liquefiedgas from a loading terminal to an unloading terminal utilizing aplurality of vessels having a first cryogenic tank means for said firstliquefied gas and a second cryogenic tank means for said secondliquefied gas wherein said cryogenic tank means previously stored saidfirst liquefied gas which comprises:

a. determining the quantity of said second liquefied gas to be stored atsaid loading terminal at the estimated time of arrival of said vessel;and

b. preparing en route said second cryogenic tank means for the loadingof said second liquefied gas should the amount be sufficient to fillsaid second cryogenic tank means, the preparation of said secondcryogenic tank means including the introduction of a higher boiling gasformed from said second liquefied gas into said second cryogenic tank39'. The method as defined in claim 38 wherein said first and secondliquefied gases are LNG and liquefied ethylene. and wherein the ratio ofthe volumetric capacity of said first cryogenic tank means to thevolumetric capacity of said second cryogenic tank means is at leastabout 4:1.

1. A process for the re-liquefaction of a gas evolved from a firstliquefied gas stored at about atmospheric pressure in a first cryogenictank means proximate to a second cryogenic tank means storing a secondliquefied gas at about atmospheric pressure wherein the first liquefiedgas has a boiling point higher than said second liquefied gas whichcomprises: a. withdrawing a gas from said first cryogenic tank means; b.withdrawing a gas from said second cryogenic tank means; and c. passingsaid gases in indirect heat transfer relationship therebetween toliquefy the gas withdrawn from said first cryogenic tank means.
 2. Theprocess as defined in claim 1 wherein each tank of said cryogenic tankmeans are of like capacity and are positioned on a vessel.
 3. Theprocess as defined in claim 2 wherein said higher boiling liquefied gasis returned to said cryogenic tank means storing said higher boilingliquefied gas.
 4. The process as defined in claim 2 wherein saidliquefied gases withdrawn from said respective cryogenic tank means arecompressed prior to step (c).
 5. The process as defined in claim 2wherein the gas from said second cryogenic tank means recovered fromstep (c) is passed to a propulsion unit of said vessel.
 6. The processas defined in claim 2 wherein said second liquefied gas is LNG.
 7. Theprocess as defined in claim 6 wherein said first liquefied gas isliquefied ethylene and wherein the ratio of the volumetric capacity ofsaid second cryogenic tank means to the volumetric capacity of saidfirst cryogenic tank means is at least about 4:1.
 8. The process asdefined in claim 6 wherein said first liquefied gas is liquefied ethaneand wherein the ratio of the volumetric capacity of said secondcryogenic tank means to the volumetric capacity of said first cryogenictank means is between about 3.0:1 to 3.5:1.
 9. The process as defined inclaim 6 wherein said first liquefied gas is LPG and wherein the ratio ofthe volumetric capacity of said second cryogenic tank means to thevolumetric capacity of said first cryogenic tank means is about 2:1. 10.Apparatus for the re-liquefaction of a gas evolved from a firstliquefied gas stored in first cryogenic tank means proximate to a secondcryogenic tank means storing a second liquefied gas wherein saidliquefied gases are stored at about atmospheric pressure and whereinsaid liquefied gas have different boiling points which comprises: a.first conduit means for withdrawing a gas from said first cryogenic tankmeans; b. a second conduit means for withdrawing a gas from said secondcryogenic tank means; and c. heat transfer means for passing said gasesin indirect heat transfer relationship to re-liquefy the gas withdrawnfrom the cryogenic tank means storing the higher boiling liquefied gas.11. The apparatus as defined in claim 10 including a third conduit meansfor passing said liquefied gas from said heat transfer means to saidcryogenic tank means storing said higher boiling liquefied gas.
 12. Theapparatus as defined in claim 10 wherein said re-liquefaction apparatusis disposed on a vessel.
 13. The apparatus as defined in claim 12including compressor means disposed in said first and second conduitmeans.
 14. The apparatus as defined in claim 12 including a fourthconduit means for the gas withdrawn from said cryogenic tank meansstoring a lower boiling liquefied gas to propulsion means for saidvessel.
 15. Apparatus for modifying a cryogenic tank on a vessel havinga plurality of cryogenic tanks for storing at about atmospheric pressureliquefied gases having different boiling points wherein said modifiedtank is to store a higher boiling liquefied gas which comprises: a. heattransfer means positioned on said vessel; b. a first conduit meansdisposed between the vapor space of said one of said plurality ofcryogenic tanks and said heat transfer means; and c. a second conduitmeans disposed between the vapor space of said remaining tanks and saidheat transfer means, said heat transfer means providing for the passagein indirect heat transfer relationship of vapors in each of said conduitmeans.
 16. The apparatus as defined in claim 15 including a thirdconduit means disposed between said heat transfer means and said one ofsaid plurality of tanks.
 17. The apparatus as defined in claim 16including a fourth conduit means disposed between said heat transfermeans and a liquefied gas holding tank.
 18. The apparatus as defined inclaim 17 including a fifth conduit means disposed between said holdingtank and said one of said plurality of cryogenic tanks, and wherein asecond heat transfer means is provided in said fifth conduit means. 19.A method for modifying a cryogenic tank on a vessel having a pluralityof cryogenic tanks to store at about atmospheric pressure liquefiedgases having different boiling points wherein said modified tank is tostore a higher boiling liquefied gas of said liquefied gases whichcomprises: a. positioning a heat transfer means on said vessel; b.providing a first conduit means from the vapor space of said one of saidplurality of tanks to said heat transfer means; and c. providing asecond conduit means between the vapor space of said remaining tanks andsaid heat transfer means, said heat transfer means providing for thepassage of said gases in each of said cOnduit means in indirect heattransfer relationship therebetween.
 20. The method as defined in claim19 wherein a third conduit means is provided between said heat transfermeans and said one of said plurality of tanks.
 21. The method as definedin claim 20 wherein a fourth conduit means is provided between said heattransfer means and a liquefied gas holding tank.
 22. The method asdefined in claim 21 wherein a fifth conduit means is provided betweensaid holding tank and said one of said plurality of tanks and wherein asecond heat transfer means is included in said fifth conduit means. 23.The method as defined in claim 19 wherein a compressor means is providedin said first and second conduit means.
 24. A process for thepreparation of a first cryogenic tank means for the storage of a firstliquefied gas on a vessel having a second cryogenic tank means for thestorage of said first liquefied gas wherein said first cryogenic tankmeans previously stored a second liquefied gas having a higher boilingpoint and wherein residual quantities of said second liquefied gasremain in said first cryogenic tank means after unloading said secondliquefied gas therefrom, which comprises: a. withdrawing a gas evolvedfrom the residual portions of gas in said second cryogenic tank means;b. introducing a portion of said evolved gas of step (a) into said firstcryogenic tank means; c. withdrawing a gas from said first cryogenictank means; d. passing said gas of step (c) in indirect heat transferrelationship with the remaining portion of gas from step (a) to liquefysaid gas of step (c); and e. passing said liquefied gas recovered fromstep (d) to a storage holding zone.
 25. The process as defined in claim24 wherein said first liquefied gas is LNG.
 26. The process as definedin claim 25 wherein said second liquefied gas is liquefied ethylene, andwherein the ratio of the volumetric capacity of said second cryogenictank means to the volumetric capacity of said first cryogenic tank meansis at least about 4:1.
 27. The process as defined in claim 24 whereinsaid preparation is effected during transit of said vessel. 28.Apparatus for the preparation of a first cryogenic means for the storageof a first liquefied gas on a vessel having a second cryogenic tankmeans for the storage of said first liquefied gas wherein said firstcryogenic tank means previously stored a second liquefied gas having aboiling point higher than said first liquefied gas which comprises: a. afirst conduit means for withdrawing a gas from said first cryogenic tankmeans; b. a second conduit means for withdrawing a gas from said secondcryogenic tank means; c. a third conduit means for passing a portion ofsaid gas withdrawn from said second cryogenic tank means to said firstcryogenic tank means; and d. heat transfer means for passing in indirectheat transfer relationship the gas in said first conduit means and theremaining portion of gas in said second conduit means to liquefy saidgas in said first conduit means.
 29. The apparatus as defined in claim28 including a fourth conduit means for passing liquefied gas to astorage tank.
 30. The apparatus as defined in claim 28 wherein the ratioof the volumetric capacity of said second cryogenic tank means to thevolumetric capacity of said first cryogenic tank means is at least about4:1 and wherein said first and second liquefied gases are LNG andliquefied ethylene, respectively.
 31. A process for the preparation of afirst cryogenic tank means for the storage of a first liquefied gas on avessel having a second cryogenic tank means for the storage of a secondliquefied gas having a boiling point lower than said first liquefied gaswherein said first cryogenic tank means previously stored said firstliquefied gas, which comprises: a. introducing into said first cryogenictank means a gas formed from said first liquefied gas; b. withdrawing agas from said first cRyogenic tank means and passing said gas inindirect heat transfer relationship with a gas evolved from said secondliquefied gas in said second cryogenic tank means to condense higherboiling gas contained therein; and c. discontinuing the flow of said gasinto said first cryogenic tank means after a predetermined period oftime and thereafter introducing a first portion of said first liquefiedgas into said first cryogenic tank means.
 32. The process as defined inclaim 31 wherein said first and second liquefied gases are liquefiedethylene and LNG, respectively, and wherein the ratio of the volumetriccapacity of said second cryogenic tank means to the volumetric capacityof said first cryogenic tank means is at least about 4:1.
 33. Apparatusfor the preparation of a first cryogenic tank means for the storage of afirst liquefied gas on a vessel having a second cryogenic tank means forthe storage of a second liquefied gas having a lower boiling point thansaid first liquefied gas and wherein said first cryogenic tank meanspreviously stored said second liquefied gas which comprises: a. astorage tank on said vessel for said first liquefied gas; b. a firstconduit means for introducing into said first cryogenic tank means a gasfrom said storage tank; c. a second conduit means for withdrawing gasfrom said first cryogenic tank means; d. a third conduit means forwithdrawing a gas from said second cryogenic tank means; and e. a heattransfer means for passing in indirect heat transfer relationship thegas in each of said second and third conduit means to condense anyhigher boiling gas in said second conduit means.
 34. The apparatus asdefined in claim 33 wherein the ratio of the volumetric capacity of saidsecond cryogenic tank means to the volumetric capacity of said firstcryogenic tank means is at least about 4:1 and wherein said first andsecond liquefied gases are liquefied ethylene and LNG, respectively. 35.A method of transporting a first liquefied gas from a loading terminalto an unloading terminal utilizing a plurality of vessels having a firstcryogenic tank means for said first liquefied gas and a second cryogenictank means for a second liquefied gas having a boiling point higher thansaid first liquefied gas which comprises: a. determining the quantity ofsaid second liquefied gas to be stored at said loading terminal at theestimated time of arrival of said vessel; and b. preparing en route saidsecond cryogenic tank means for the loading of said first liquefied gasshould the amount of said second liquefied gas be insufficient to fillsaid second cryogenic tank means, the preparation of said secondcryogenic tank means including the introduction of a portion of naturalgas in said first cryogenic tank means into said second cryogenic tankmeans and the re-liquefaction of higher boiling gas withdrawn from saidsecond cryogenic tank means by the passage of the gas withdrawn fromsaid second cryogenic tank means in indirect heat transfer relationshipto a portion of natural gas withdrawn from said second cryogenic tankmeans.
 36. The method as defined in claim 35 wherein said first andsecond liquefied gases are LNG and liquefied ethylene, and wherein theratio of the volumetric capacity of said first cryogenic tank means tothe volumetric capacity of said second cryogenic tank means is at leastabout 4:1.
 37. A method of transporting a first liquefied gas and asecond liquefied gas having a boiling point higher than said firstliquefied gas from a loading terminal to an unloading terminal utilizinga plurality of vessels having a first cryogenic tank means for saidfirst liquefied gas and a second cryogenic tank means for said secondliquefied gas wherein said cryogenic tank means previously stored saidfirst liquefied gas which comprises: a. determining the quantity of saidsecond liquefied gas to be stored at said loading terminal at theestimated time of arrival of said vessEl; and b. preparing en route saidsecond cryogenic tank means for the loading of said second liquefied gasshould the amount be sufficient to fill said second cryogenic tankmeans, the preparation of said second cryogenic tank means including theintroduction of a higher boiling gas formed from said second liquefiedgas into said second cryogenic tank means and the withdrawal of a gasfrom said second cryogenic tank means.
 38. The method as defined inclaim 37 wherein the gas withdrawn from said second cryogenic tank meansis passed in indirect heat transfer relationship to gas withdrawn fromsaid first cryogenic tank means to liquefy any higher boiling gascontained therein.
 39. The method as defined in claim 38 wherein saidfirst and second liquefied gases are LNG and liquefied ethylene, andwherein the ratio of the volumetric capacity of said first cryogenictank means to the volumetric capacity of said second cryogenic tankmeans is at least about 4:1.